Generalized scalability for video coder based on video objects

ABSTRACT

A video coding system that codes video objects as scalable video object layers. Data of each video object may be segregated in to one or more layers. A base layer contains sufficient information to decode a basic representation of the video object. Enhancement layers contain supplementary data regarding the video object that, if decoded, enhance the basic representation obtained from the base layer. The present invention thus provides a coding scheme suitable for use with decoders of varying processing power. A simple decoder may decode only the base layer of the video objects to obtain the basic representation. However, more powerful decoders may decode the base layer data of video objects and additional enhancement layer data to obtain improved decoded output. The coding scheme supports enhancement of both the spatial resolution and the temporal resolution of video object.

This application is a continuation of U.S. patent application Ser. No.12/253,307, filed Oct. 17, 2008, which is currently allowed and is acontinuation of U.S. patent application Ser. No. 11/197,700, filed Aug.4, 2005, now U.S. Pat. No. 7,457,474, which is a continuation of U.S.patent application Ser. No. 10/761,518, filed Jan. 20, 2004, now U.S.Pat. No. 6,993,201, which is a continuation of U.S. patent applicationSer. No. 10/336,709, filed Jan. 6, 2003, now U.S. Pat. No. 6,707,949,which is a continuation of U.S. patent application Ser. No. 09/814,061,filed Mar. 22, 2001, now U.S. Pat. No. 6,526,177, which is acontinuation of U.S. patent application Ser. No. 09/111,215, filed Jul.7, 1998, now U.S. Pat. No. 6,233,356, which claims priority to U.S.Provisional Patent Application Ser. No. 60/069,888 filed Jul. 8, 1997,and all of which are herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a video coding system in which imagedata is organized into video objects and coded according to scalablecoding scheme. The coding scheme provides spatial scalability, temporalscalability or both.

2. Related Art

Video coding is a field that currently exhibits dynamic change. Videocoding generally relates to any method that represents natural and/orsynthetic visual information in an efficient manner. A variety of videocoding standards currently are established and a number of other codingstandards are being drafted. The present invention relates to aninvention originally proposed for use in the Motion Pictures ExpertsGroup standard MPEG4.

One earlier video standard, known as “MPEG-2,” codes video informationas video pictures or “frames.” Consider a sequence of video informationto be coded, the sequence represented by a series of frames. The MPEG-2standard coded each frame according to one of three coding methods. Agiven image could be coded according to:

-   -   Intra-coding where the frame was coded without reference to any        other frame (known as “I-pictures”),    -   Predictive-coding where the frame was coded with reference to        one previously coded frame (known as “P-pictures”), or    -   Bi-directionally predictive coding where the frame was coded        with reference to as many as two previously coded frames (known        as “B-pictures”).

Frames are not necessarily coded in the order in which they appear underMPEG-2. It is possible to code a first frame as an I-picture then code afourth frame as a P-picture predicted from the I-picture. Second andthird frames may be coded as B-pictures, each predicted with referenceto the I- and P-pictures previously coded. A time index is provided topermit a decoder to reassemble the correct frame sequence when itdecodes coded data.

MPEG-4, currently being drafted, integrated the concept of “videoobjects” to I-, P- and B-coding. Video object based coders decompose avideo sequence into video objects. An example is provided in FIGS. 1(a)-(d). There, a frame includes image data including the head andshoulders of a narrator, a suspended logo and a background. An encodermay determine that the narrator, logo and background are three distinctvideo objects, each shown separately in FIGS. 1( b)-(d). The video codermay code each separately.

Video object-based coding schemes recognize that video objects mayremain in a video sequence across many frames. The appearance of a videoobject on any given frame is a “video object plane” or “VOP”. VOPs maybe coded as I-VOPs using intra coding techniques, as P-VOPs usingpredictive coding techniques or B-VOPs using bi-directionally predictivecoding techniques. For each VOP, additional administrative data istransmitted with the coded VOP data that provides information regarding,for example, the video objects location in the displayed image.

Coding video information on a video object-basis may improve codingefficiency in certain applications. For example, if the logo were astatic image, an encoder may code it as an initial I-VOP. However, forsubsequent frames, coding the logo as a P- or B-VOP would yield almostno image data. The P- or B-coding essentially amounts to an“instruction” that the original image information should be redisplayedfor successive frames. Such coding provides improved coding efficiency.

One goal of the MPEG-4 standard is to provide a coding scheme that maybe used with decoders of various processing power. Simple decodersshould be able to decode coded video data for display. More powerfuldecoders should be able to decode the coded video data and obtainsuperior output such as improved image quality or attachedfunctionalities. As of the priority date of this application, no knownvideo object-based coding scheme provides such flexibility.

MPEG-2 provides scalability for its video picture-based coder. However,the scalability protocol defined by MPEG-2 is tremendously complicated.Coding of spatial scalability, where additional data for VOPs is codedinto an optional enhancement layer, is coded using a first protocol.Coding of temporal scalability, where data of additional VOPs is codedin the enhancement layer, is coded using a second protocol. Eachprotocol is separately defined from the other and requires highlycontext specific analysis and complicated lookup tables in a decoder.The scalability protocol of the MPEG-2 is disadvantageous because itscomplexity makes it difficult to implement. Accordingly, there is afurther need in the art for a generalized scalability protocol.

SUMMARY OF THE INVENTION

The present invention provides a video coding system that codes videoobjects as video object layers. Data of each video object may besegregated into one or more layers. A base layer contains sufficientinformation to decode a basic representation of the video object.Enhancement layers contain supplementary data regarding the video objectthat, if decoded, enhance the basic representation obtained from thebase layer. The present invention thus provides a coding scheme suitablefor use with decoders of varying processing power. A simple decoder maydecode only the base layer to obtain the basic representation. However,more powerful decoders may decode the base layer data and additionalenhancement layer data to obtain improved decoded output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-(d) provide an example of video data and video objects thatmay be derived therefrom.

FIG. 2 is an organizational chart illustrating a video coding hierarchyestablished by the present invention.

FIG. 3 illustrates an object based video coder constructed in accordancewith an embodiment of the present invention.

FIG. 4 is a block diagram of a video object encoder constructed inaccordance with an embodiment of the present invention.

FIG. 5 illustrates an application of temporal scalability provided bythe present invention.

FIG. 6 illustrates an application of spatial scalability provided by thepresent invention.

FIG. 7 is a block diagram of a video object decoder constructed inaccordance with an embodiment of the present invention.

FIG. 8 is a block diagram of a scalability preprocessor constructed inaccordance with an embodiment of the present invention.

FIG. 9 is a block diagram of an enhancement layer encoder constructed inaccordance with an embodiment of the present invention.

FIG. 10 is a block diagram of a midprocessor constructed in accordancewith an embodiment of the present invention.

FIG. 11 is a block diagram of an enhancement layer decoder constructedin accordance with an embodiment of the present invention.

FIG. 12 is a block diagram of a scalability post-processor constructedin accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention introduces a concept of “video object layers” tothe video object-based coding scheme. Data of each video object may beassigned to one or more layers of the video object and coded. A baselayer contains sufficient information to represent the video object at afirst level of image quality. Enhancement layers contain supplementarydata regarding the video object that, if decoded, improve the imagequality of the base layer. The present invention thus provides an objectbased a coding scheme suitable for use with decoders of varyingprocessing power. A simple decoder may decode only the base layer ofobjects to obtain the basic representation. More powerful decoders maydecode the base layer data and additional enhancement layer data ofobjects to obtain improved decoded output.

FIG. 2 illustrates an organizational scheme established by the presentinvention. An image sequence to be coded is a video session. The videosession may be populated by a number of video objects. Each video objectmay be populated by one or more video object layers. A video objectlayer is an organizational artifact that represents which part of thecoded bitstream output by the video coder carries certain imageinformation related to the video object. For example, base layer datamay be assigned to a first video object layer (layers VOL1 for eachvideo object VOØ, VO1 and VO2 in FIG. 2). Enhancement layer data may beassigned to a second video object layer, such as VOL2 in each of VO1 andVO2. The video object layers are themselves populated by video objectplanes.

Enhancement layers need not be provided for every video object. Forexample, FIG. 2 illustrates a video session that provides only a singlevideo object layer for video object VOØ.

There is no limit to the number of video object layers that may beprovided for a single video object. However, each video object layeradded to a video object will be associated with a certain amount ofadministrative information required to code the video object layer. Theoverhead administrative data can impair coding efficiency.

FIG. 3 illustrates a video coding system constructed in accordance withan embodiment of the present invention. The coding system includes anencoder 100 and a decoder 200 separated by a channel 300. The encoder100 receives input video objects data and codes the video objects dataaccording to the coding scheme described above with respect to FIG. 2.The encoder 100 outputs coded data to the channel 300. The decoder 200receives the coded data from the channel 300 and decodes it usingtechniques complementary to those used at the encoder 100. The decoderoutputs decoded video data for display, storage or other use.

The channel 300 may be a real time data medium in which coded dataoutput from the encoder 100 is routed directly to the decoder 200. Assuch, the channel 300 may be represented by a data communication channelprovided by the Internet, a computer network, a wireless data network ora telecommunication network. The channel 300 may also be a storagemedium, such as a magnetic, optical or electrical memory. In theseapplications, the encoder 100 and decoder 200 need not workcontemporaneously. The encoder 100 may store coded data in the channel300 where the coded data may reside until retrieved by the decoder 200.

The encoder 100 includes a video object segmenter/formatter 400,plurality of 20 video object encoders 500 a-n and a systems multiplexer(“MUX”) 600. In a typical application, the encoder 100 may be amicroprocessor or digital signal processor that is logically dividedinto these components 400-600 by program instructions. Alternatively,the components 400-600 may be populated by hardware components adaptedto perform these functions.

The video objects segmenter/formatter 400 receives input video data andidentifies video objects therefrom. The process of decomposing an imagesequence into video objects is well known and described in “Coding ofMoving Pictures and Video,” ISO/IEC 14496-2 (July 1997). The videoobject segmenter/formatter 400 outputs VOP data to each of the videoobject encoders 500 a-n.

The video object encoders 500 a-n receive the VOP data of theirrespective video objects and code the VOP data according to thestructure shown in FIG. 2. That is, the video object encoder (say, 500a) determines how many video object layers to use in coding the videoobject data. It determines what part of the input VOP data is coded asbase layer data and what part is coded as enhancement layer data. Thevideo object encoder codes the base layer data and any enhancement layerdata as coded VOPs of each video object layer. It outputs coded videoobject data to the MUX 600.

The MUX 600 organizes the coded video object data received from each ofthe video object encoders 500 into a data stream and outputs the datastream to the channel 300. The MUX 600 may merge data from othersources, such as audio coders (not shown), graphics coder (not shown),into the unitary signal stream.

The decoder 200 includes a systems demultiplexer (“DEMUX”) 700, aplurality of video object decoders 800 a-n and a video objectscompositor 900. As with the encoder 100, the decoder 200 may be amicroprocessor or digital signal processor that is logically dividedinto these components 700-900 by program instructions. Alternatively,the components 700-900 may be populated by hardware components adaptedto perform these functions.

The DEMUX 700 retrieves the unitary coded signal from the data streamchannel 300. It distinguishes the coded data of the various videoobjects from each other. Data for each video object is routed to arespective video object decoder 800 a-n. Other coded data, such asgraphics data or coded audio data, may be routed to other decoders (notshown).

The video object decoders 800 a-n decode base layer data and anyenhancement layer data using techniques complementary to those appliedat the video object encoders 500 a-n. The video object decoders 800 a-noutput decoded video objects.

The video objects compositor 900 assembles a composite image from thedecoded VOP data of each video object. The video objects compositor 900outputs the composite image to a display, memory or other device asdetermined by a user.

FIG. 4 is a block diagram of a video object encoder 500 a of the presentinvention. The video object encoder includes a scalability pre-processor510, a base layer encoder 520, a midprocessor 530, an enhancement layerencoder 540 and an encoder multiplexer 550. Again, the components of thevideo object encoder 500 a may be provided in hardware or may be logicaldevices provided in a microprocessor or a digital signal processor.

VOP data of a video object is input to the scalability pre-processor510. The scalability pre-processor 510 determines which data is to becoded in the base layer and which data is to be coded in the enhancementlayer. It outputs a first set of VOPs to the base layer encoder 520 anda second set of VOPs to the enhancement layer encoder 540.

The base layer encoder 520 codes base layer VOPs according toconventional techniques. Such coding may include the nonscalable codingtechniques of the MPEG-4 standard. Base layer VOPs are coded by intracoding, predictive coding or bi-directionally predictive coding andoutput on line 522 to the encoder multiplexer MUX 550. The base layerencoder also outputs locally decoded VOPs on line 524. The base layerencoder obtains locally decoded VOPs by decoding the coded base layerdata. Effectively, the locally decoded VOPs mimic decoded base layerdata that is obtained at the decoder 200.

The midprocessor 530 receives the locally decoded VOPs and depending onits mode of operation, outputs up sampled, down sampled or unchanged VOPdata to the enhancement layer encoder 540.

The enhancement layer encoder 540 receives VOP data from the scalabilitypreprocessor 510 and locally decoded VOP data possibly having beenmodified by the midprocessor 530. The enhancement layer encoder 540codes the VOP data received from the scalability preprocessor using thelocally decoded VOP data as a basis for prediction. It outputs codedenhancement layer data to the encoder multiplexer 550.

The encoder multiplexer MUX 550 outputs coded base and enhancement layervideo object data from the video object encoder.

FIG. 5 illustrates an example of object based temporal scalability thatmay be achieved by the present invention. There, a first sequence ofVOPs 1010, 1030, 1050, . . . are coded by the base layer encoder 520 anda second sequence of VOPs 1020, 1040, . . . are coded by the enhancementlayer encoder 540. In time order, the VOPs appear in the order: 1010,1020, 1030, 1040, 1050, . . . .

The base layer encoder 520 codes VOP 1010 first as an I-VOP. Second, itcodes VOP 1050 as a P-VOP using VOP 1010 as a basis for prediction.Third, it codes VOP 1030 as a B-VOP using VOPs 1010 and 1050 as basesfor prediction.

The enhancement layer encoder 540 codes VOP 1020 using base layerlocally decoded VOPs 1010 and 1030 as bases for prediction. It alsocodes VOP 1040 using base layer locally decoded VOPs 1030 and 1050 asbases for prediction. Although not shown in FIG. 5, an enhancement layerVOP (such as 1040) can look to another enhancement layer VOP as a basisfor prediction. For example, VOP 1040 could be coded using VOPs 1020 asa basis for prediction.

On decoding, a simple decoder decodes only the coded base layer data. Itdecodes and displays VOPs 1010, 1030, 1050, . . . providing a videosequence for display having a first frame rate. A power decoder,however, that decodes both base layer and enhancement layer data obtainsthe entire VOP sequence 1010, 1020, 1030, 1040, 1050, . . . . It decodesa video sequence having a higher frame rate. With a higher frame rate,an observer would perceive more natural motion.

FIG. 6 illustrates an example of object based spatial scalability thatmay be achieved by the present invention. There, VOPs 1110-1140 arecoded by the base layer encoder 520. Spatially, larger VOPs 1210-1240are coded by the enhancement layer encoder 540. Enhancement layer VOPs1210-1240 coincide, frame for frame, with the base layer VOPs 1110-1140.

The base layer encoder 520 codes the base layer VOPs in the order 1110,1130, 1120, . . . . VOP 1110 is coded as an I-VOP. VOP 1130 is coded asa P-VOP using VOP 1110 as a basis for prediction. VOP 1120 is codedthird as a B-VOP using VOPs 1110 and 1130 as a basis for prediction. VOP1140 is coded sometime thereafter using VOP 1130 and another VOP (notshown) as a basis for prediction.

The enhancement layer encoder 540 codes the enhancement layer VOPs inthe order 1210, 1220, 1230, 1240, . . . . As shown in FIG. 6, VOP 1210is a P-VOP coded using VOP 1110 as a basis for prediction. VOP 1220 iscoded as a B-VOP using base layer VOP 1120 and enhancement layer VOP1210 as a basis for prediction. VOPs 1230 and 1240 are coded in a mannersimilar to VOP 1220; they are coded as B-VOPs using the temporallycoincident VOP from the base layer and the immediately previousenhancement layer VOP as a basis for prediction.

On decoding, a simple decoder that decodes only the coded base layerdata obtains the smaller VOPs 1110-1140. However, a more powerfuldecoder that decodes both the coded base layer data and the codedenhancement layer data obtains a larger VOP. On display, the decodedvideo object may be displayed as a larger image or may be displayed at afixed size but may be displayed with higher resolution.

Scalability also provides a graceful degradation in image quality in thepresence of channel errors. In one application, the coded base layerdata may be supplemented with error correction coding. As is known,error correction coding adds redundancy to coded information. Errorcoded signals experience less vulnerability to transmission errors thansignals without error coding. However, error coding also increases thebit-rate of the signal. By providing error correction coding to thecoded base layer data without providing such coding to the codedenhancement layer data, an intermediate level of error protection isachieved without a large increase in the bit rate. Enhancement layerVOPs are not error coded, which would otherwise reduce the transmittedbit rate of the unified signal. When channel errors occur, the codedbase layer data is protected against the errors. Thus, at least a basicrepresentation of the video object is maintained. Graceful signaldegradation is achieved in the presence of channel errors.

FIG. 7 illustrates a block diagram of a video object decoder 800 aconstructed in accordance with an embodiment of the present invention.The video object decoder 800 a includes a decoder demultiplexer (DEMUX)810, a base layer decoder 820, a midprocessor 830, an enhancement layerdecoder 840 and a scalability post-processor 850. The components of thevideo object decoder 800 a may be provided in hardware or may be logicaldevices provided in a microprocessor or a digital signal processor.

The DEMUX 810 receives the coded video object data from the systemdemultiplexer 700 (FIG. 3). It distinguishes coded base layer data fromcoded enhancement layer data and routes each type of data to the baselayer decoder 820 and enhancement layer decoder 840 respectively.

The base layer decoder 820 decodes the coded base layer data to obtainbase layer VOPs. It outputs decoded base layer VOPs on output 822. Inthe absence of channel errors, the decoded base layer VOPs shouldrepresent identically the locally decoded VOPs output on line 524 fromthe base layer, encoder 520 to the midprocessor 530 (FIG. 4). Thedecoded base layer VOPs are input to the scalability post processor 850and to the midprocessor 830 (line 524).

The decoder midprocessor 830 operates identically to the encodermidprocessor 530 of FIG. 4. If midprocessor 530 had up sampled locallydecoded VOPs, midprocessor 830 up samples the decoded base layer VOPs.If midprocessor 530 had down sampled or left unchanged the locallydecoded VOPs, midprocessor 830 also down samples or leaves unchanged thedecoded base layer VOPs. An output of the midprocessor 830 is input tothe enhancement layer decoder 840.

The enhancement layer decoder 840 receives coded enhancement layer datafrom the DEMUX 810 and decoded base layer data (possibly modified) fromthe midprocessor 830. The enhancement layer decoder 840 decodes thecoded enhancement layer data with reference to the decoded base layerdata as necessary. It outputs decoded enhancement layer VOPs to thescalability post-processor 850.

The scalability post-processor 850 generates composite video object datafrom the decoded base layer data and the decoded enhancement layer data.In the case of temporal scalability, the scalability post-processor 850reassembles the VOPs in the correct time ordered sequence. In the caseof spatial scalability, the scalability post-processor outputs thedecoded enhancement layer-data. The decoded base layer data isintegrated into the decoded enhancement layer VOPs as part of thedecoding process.

FIG. 8 illustrates a block diagram of the scalability pre-processor 510(FIG. 4). The scalability pre-processor 510 includes a temporaldecimator 511, a horizontal and vertical decimator 512 and a temporaldemultiplexer 513. It can perform spatial resolution reduction(horizontal and/or vertical) and temporal resolution reduction bydropping intermediate pictures or VOPs as necessary. VOPs input to thescalability pre-processor are input on line 514. The scalabilitypre-processor outputs VOPs to the base layer decoder on line 515 andother VOPs to the enhancement layer decoder on line 516.

The temporal decimator 511 reduces the VOP rate of both the base layerand the enhancement layer by dropping predetermined VOPs.

The temporal demultiplexer is used for temporal scalability. For a givenVOP input to it, the temporal demultiplexer 513 routes it to either thebase layer decoder (over output 515) or to the enhancement layer decoder(over output 516).

The horizontal and vertical decimator 512 may be used for spatialscalability. Each VOP input to the scalability pre-processor (or, atleast, those output from the temporal decimator) is output directly tothe enhancement layer decoder over line 516. The VOPs are also input tothe horizontal and vertical decimator where image data of each VOP isremoved to shrink them. The shrunken VOPs output from the horizontal andvertical decimator are output to the base layer encoder over line 515.

FIG. 9 is a block diagram of an enhancement layer encoder 540 for videoobjects constructed in accordance with the present invention. Theenhancement layer encoder 540 includes a VOP Motion Compensated DCTEncoder 541, a VOP Interlayer Motion Estimator 542 (“VIME”) and a VOPInterlayer Motion Compensated Predictor 543. It receives the enhancementlayer VOPs from the scalability pre-processor 510 at input 544 and thelocally decoded base layer VOPs (possibly modified) at input 545: Theenhancement layer encoder outputs the coded enhancement layer data onoutput 546.

The enhancement layer encoder 540 receives the enhancement layer VOPsfrom the scalability pre-processor 510 on input 544. They are input tothe VOP Motion Compensated DCT Encoder 541 and to the VOP InterlayerMotion Estimator 542. The VOP Motion Compensated DCT Encoder 541 is amotion compensated transform encoder that is adapted to accept apredicted VOP and motion vectors as inputs. The motion vectors aregenerated by VIME 542, a normal motion estimator that has been adaptedto accept enhancement layer VOPs from input 544.

VIME 542 performs motion estimation on an enhancement layer VOP withreference to a locally decoded base layer VOP. It outputs motion vectorsto the VOP Interlayer Motion Compensated Predictor 543 and, selectively,to the VOP Motion Compensated DCT Encoder 541.

The VOP Interlayer Motion Compensated Predictor 543 is a normal motioncompensated predictor that operates on the locally decoded base layerVOPs received from the midprocessor 530. It obtains a prediction fromone or two possible sources of prediction. In a first prediction,prediction is made with reference to a first VOP. In a secondprediction, prediction is made with reference to a second VOP. A thirdprediction obtains an average of the first and second predictions. Thesource of predictions, the first and second VOPs, may be located ineither the base layer or enhancement layer. Arrows in FIGS. 5 & 6illustrate exemplary prediction directions.

In an MPEG-4 system image data of video objects is organized into blocksof image data. Prediction according to the three predictions describedabove may be performed on a block by block basis. Thus a first block ofa VOP may be predicted using prediction 1 (First VOP), a second blockmay be predicted using prediction 2 (second VOP), and a third block maybe predicted using prediction 3 (both VOPs). In the embodiment, thefirst and second VOPs are properly viewed as possible sources forprediction because they may be used as sources for prediction but arenot necessary used.

The VOP Interlayer Motion Compensated Predictor 543 outputs predictedVOPs. The output of the VOP Interlayer Motion Compensated Predictor 543or the locally decoded base layer VOPs are input to the VOP MotionCompensated DCT Encoder 541.

FIG. 10 is a block diagram of a midprocessor 530, 830 constructed inaccordance with an embodiment of the present invention. The midprocessor530, 830 includes a horizontal interpolator 531 and a verticalinterpolator 532 on a first processing path, a horizontal decimator 533and a vertical decimator 534 on a second processing path and a third,shunt path 535. It receives VOPs on input 536 and outputs VOPs on anoutput 537.

The horizontal interpolator 531 and vertical interpolator 532 areenabled when the midprocessor 530, 830 operates in an up sampling mode.For each VOP, the horizontal interpolator 531 and vertical interpolator532 enlarge the VOP and calculate image data for data point(s) betweenoriginal data points.

The horizontal decimator 533 and vertical decimator 534 are enabled whenthe midprocessor 530, 830 operates in down sampling mode. The horizontaldecimator 533—and vertical decimator 534 reduce the VOP and remove imagedata for certain of the original data points.

The shunt path 535 outputs untouched the VOPs input to the midprocessor530, 830.

FIG. 11 is a block diagram of the enhancement layer decoder of videoobjects 840 of FIG. 7. The enhancement layer decoder 840 includes a VOPMotion Compensated DCT Decoder 841 and a VOP Interlayer MotionCompensated Predictor 842. The coded enhancement layer data is input tothe enhancement layer decoder on input 843. Decoded base layer VOPsreceived from the midprocessor 830 are input to the enhancement layerdecoder on input 844. The enhancement layer decoder 840 outputs decodedenhancement layer VOPs on output 845.

The VOP Motion Compensated DCT Decoder 841 decodes motion vectors aswell as the prediction mode from the coded enhancement layer data andoutputs them to the VOP Interlayer Motion Compensated Predictor 842along with decoded enhancement layer previous VOP. The VOP InterlayerMotion Compensated Predictor 842 also receives the decoded base layerVOPs from line 844. The VOP Interlayer Motion Compensated Predictor 842outputs predicted VOPs back to the VOP Motion Compensated DCT Decoder841. Based upon either the enhanced layer previous decoded VOPs or thedecoded base layer VOPs, or their combination, the VOP MotionCompensated DCT Decoder 841 generates the decoded enhancement layerVOPs. Among the combinations allowed at the encoder are one-half ofprevious decoded enhancement layer VOP and one-half of the base layerVOP, as well as one-half of a previous and a next decoded VOP of baselayer.

FIG. 12 is a block diagram of the scalability post-processor 850. Itincludes a temporal multiplexer 851 and a temporal interpolator 852. Thescalability post-processor 850 receives decoded base layer data on input853 and decoded enhancement layer VOPs on input 854. It outputscomposite video object data on output 855.

The temporal multiplexer 851 reassembles the VOPs from the base layerand the enhancement layer into a single stream of VOPs. The temporalinterpolator 852 is used for temporal scalability to rearrange VOPs intothe correct time ordered sequence. For spatial scalability, the decodedbase layer VOPs may be ignored; the decoded enhancement layer databypasses the temporal multiplexer 851.

The temporal interpolator 852 increases the frame rate of the VOPs in amanner that complements the temporal decimator 511 of the video objectencoder 500 a (FIG. 8). If the temporal decimator 511 was bypassed forencoding, the temporal interpolator 852 may be bypassed during decoding.

As has been shown, the present invention provides a system providingscalability, either temporal scalability, spatial scalability or both.VOPs are separated into base layer VOPs and enhancement layer VOPs andcoded as such. On decoding, a specific decoder may decode the coded baselayer data with or without the coded enhancement layer data, dependingon it processing power and channel conditions.

The present invention also provides a general scalability syntax whilecoding.

Generalized scalability allows predictions to be correctly formed at thedecoder by embedding the necessary codes indicating the specific type oftemporal scalability or spatial scalability to be derived. The referenceVOPs for prediction are selected by reference_select_code as describedin Tables 1 and 2. In coding P-VOPs belonging to an enhancement layer,the forward reference can be one of the following three: the most recentdecoded VOP of enhancement layer, the most recent VOP of the lower layerin display order, or the next VOP of the lower layer in display order.

In B-VOPs, the forward reference can be one of the two: the most recentdecoded enhancement VOP or the most recent lower layer VOP in displayorder. The backward reference can be one of the three: the temporallycoincident VOP in the lower layer, the most recent lower layer VOP indisplay order, or the next lower layer VOP in display order.

TABLE 1 Prediction Reference Choices For P-VOPs in The Object-BasedTemporal Scalability Ref_select_code Forward Prediction Reference 00Most recent decoded enhancement VOP belonging to the same layer. 01 Mostrecent VOP in display order belonging to the reference layer. 10 NextVOP in display order belonging to the reference layer. 11 Temporallycoincident VOP in the reference layer (no motion vectors)

TABLE 2 Prediction Reference Choices For B-VOPs In The Case OfScalability ref_select Backward code Forward Temporal Reference TemporalReference 00 Most recent decoded enhancement Temporally coincident VOPof the same layer VOP in the reference layer (no motion vectors) 01 Mostrecent decoded enhancement Most recent VOP in VOP of the same layer.display order belonging to the reference layer. 10 Most recent decodedenhancement Next VOP in display VOP of the same layer. order belongingto the reference layer. 11 Most recent VOP in display order Next VOP indisplay belonging to the reference layer. order belonging to thereference layer.

The enhancement layer can contain P or B-VOPs, however, in scalabilityconfigurations of FIG. 4 and FIG. 5, the B-VOPs in the enhancement layerbehave more like P-VOPs at least in the sense that a decoded B-VOP canbe used to predict the following P or B-VOPs.

When the most recent VOP in the lower layer is used as reference, thisincludes the VOP that is temporally coincident with the VOP in theenhancement layer. However, this necessitates use of lower layer formotion compensation which requires motion vectors.

If the coincident VOP in the lower layer is used explicitly asreference, no motion vectors are sent and this mode can be used toprovide spatial scalability. Spatial scalability in MPEG-2 usesspatio-temporal prediction, which is accomplished as per FIG. 5 moreefficiently by simply using the three prediction modes: forwardprediction (prediction direction 1), backward prediction (predictiondirection 2), interpolated prediction (prediction directions 1 and 2)available for B-VOPs.

Since the VOPs can have a rectangular shape (picture) or an irregularshape, both the traditional as well as object based temporal and spatialscalabilities become possible. We now provide some details by whichscalability can be accomplished for arbitrary shaped VOPs by extendingthe technique of chroma-keying known in the art. Normally, scalablecoding of arbitrary shaped objects requires explicit transmission ofshape information of each VOP, however, by use of a simpler technique ofchroma-keying in which only rectangular VOPs containing arbitrary shapedVOP are coded such that in the region outside of arbitrary shape ofinterest a key color (not present anywhere in the VOP) is inserted bythe encoder and specified in the bitstream allowing deletion by thedecoder, the only caveat is that the key color insertion/deletion isperformed not only on arbitrary shape VOPs of lower (here, a base) layerbut also in enhancement layer. Thus it becomes possible at the decoderto recover VOPs of scalable arbitrary shape since coding is reallyperformed on rectangular VOP windows in the same manner as coding ofpictures.

The class hierarchy introduced in FIG. 2 can be used to implement apractical bitstream representation that may allow ease of access forobject manipulation and editing functionalities. For illustrativepurposes, they are described with reference to syntax elements from“MPEG-4 Video Verification Model Version 2.1,” ISO/IEC ITC1/SC29/WG11,MPEG 96/776 (March 1996) (herein, “VM 2.1”). Tables 3-6 illustrate byexample some bitstream details of video syntax class and meaning ofvarious syntax elements in each class, particularly for reorganized ornew syntax elements.

TABLE 3 Video Session No. of Syntax Bits VideoSession( ){video_session_start_code 32 do { do { VideoObject( ) } while (nextbits() = = video_object_start_code) if (nextbits( ) ! = session_end_code)video_session_start_code 32  } while (nextbits( ) ! =video_session_end_code)  video_session_end_code 32 }

TABLE 4 Video Object No. of Syntax Bits VideoObject( ){video_object_start_code 24 + 3 object_id 5 do { VideoObjectLayer( ) }while (nextbits( ) = = video_object_layer_start_code) next_start_code( )}object_id: It uniquely identifies a layer. It is a 5-bit quantity withvalues from 0 to 31.

TABLE 5 Video Object Layer No. of Syntax Bits VideoObjectLayer( ){video_object_layer_start_code 28 layer_id 4 layer_width 10 layer height10 quant_type_sel 1 if (quant_type_sel) { load_intra_quant_mat 1 if(load_intra_quant_mat) intra_quant_mat[64] 8*64 load_nonintra_quant_mat1 if (load_nonintra_quant_mat) nonintra_quant_mat[64] 8*64 }intra_dcpred_disable 1 scalability 1 if (scalability) { ref_layer_id 4ref_layer_sampling_direc 1 hor_sampling_factor_n 5 hor_sampling_factor_m5 vert_sampling_factor_n 5 vert_sampling_factor_m 5 enhancement_type 1 }do { VideoObjectPlane( )  } while (nextbits( ) = = video_object_plane_start_code} next_start_code( ) }

layer_id: It uniquely identifies a layer. It is a 4-bit quantity withvalues from 0 to 15. A value of 0 identifies the first independentlycoded layer.

layer_width, layer_height: These values define the spatial resolution ofa layer in pixels units.

Scalability: This is a 1-bit flag that indicates if scalability is usedfor coding of the current layer.

ref_layer_id: It uniquely identifies a decoded layer to be used as areference for predictions in the case of scalability. It is a 4-bitquantity with values from 0 to 15.

ref_layer_sampling_direc: This is a 1-bit flag whose value when “0”indicates that the reference layer specified by ref_layer_id has thesame or lower resolution as the layer being coded. Alternatively, avalue of “1” indicates that the resolution of reference layer is higherthan the resolution of layer being coded resolution.

hor_sampling_factor_n, hor sampling_factor_m: These are 5-bit quantitiesin range 1 to 31 whose ratio hor_sampling_factor_n/hor_sampling_factor_mindicates the resampling needed in horizontal direction; the directionof sampling is indicated by ref_layer_sampling_direc.

vert sampling_factor_n, vert_sampling_factor_m: These are 5-bitquantities in range of 1 to 31 whose ratiovert_sampling_factor_n/vert_sampling_factor_m indicates the resamplingneeded in vertical direction; the direction of sampling is indicated byref_layer_sampling_direc.

enhancement_type: This is a 1-bit flag that indicates the type of anenhancement structure in a scalability. It has a value of í1î when anenhancement layer enhances a partial region of the base layer. It has avalue of í0î when an enhancement layer enhances entire region of thebase layer. The default value of this flag is í0î.

Other syntax elements such as quant_type_sel and intro_dcpred_disable inthe Video Object Layer have the same meaning described in VM 2.1.

TABLE 6 Video Object Plane No. of Syntax Bits VideoObjectPlane( ) { video_object_plane_start_code 32  vop_temp_ref 16  vop_visibility 1 vop_of_arbitrary_shape 1  if (vop_of_arbitrary_shape) {  vop_width 10 vop_height 10  if (vop_visibility) { vop_composition_order 5vop_hor_spatial_ref 10 marker_bit 1 vop_vert_spatial_ref 10 vop_scaling3 : : } : /* syntax to derive shapes by deleting key color */ :  } vop_coding_type 2  if (vop_coding_type = = 1 | | vop_coding_type = = 2) { vop_fcode_forward 2 if (vop_coding_type = = 2) { vop_fcode_backward 2vop_dbquant 2 } else { vop_quant 5 } if (!scalability) {separate_motion_texture 1 if (!separate_motion_texture)  combined motiontexture coding( ) else { motion_coding( ) texture coding( ) } } else { :/* syntax to derive forward and backward shapes by 1 deleting key color*/ : } ref_select_code 2 if (vop_coding_type = = 1 | | vop_coding_type == 2) { forward_temporal_ref 10 if (plane_coding_type = = 2) { marker_bit1 backward_temporal_ref 10 } } combined_motion_texture_coding( )  } }

The meaning of the syntax elements of video object planes is specifiedin VM2.1.

Accordingly, the present invention provides a video coding system andsyntax supporting generalized scalability. The system finds applicationwith limited or noisy 5 channels and with decoders of varying processingpower.

What is claimed:
 1. A video decoding system in which video objects arerecognized from video data, wherein instances of a video object at giventimes are coded as video object planes (VOPs) and VOPs are assigned toone or more video object layers, the video decoding system comprising adecoder structure, the decoder structure further comprising: a baselayer decoder having an input for VOP data associated with a first videoobject layer of the video object; a processor coupled to an output ofthe base layer decoder; and an enhancement layer decoder, having a firstinput for VOP data associated with a second video object layer of thevideo object and a second input coupled to the processor and responsiveto predictive coded VOP (P-VOP) data including a ref_select_codeincluded therein, the enhancement layer decoder decoding the P-VOP datawith reference to one of: data of a VOP most recently decoded by theenhancement layer decoder; data of a most recent VOP in a display orderdecoded by the base layer decoder; data of a next VOP in a display orderdecoded by the base layer decoder; and data of a temporally coincidentVOP by the base layer decoder.
 2. The video decoding system of claim 1,wherein the processor performs a spatial up-sampling of data receivedfrom the base layer decoder.
 3. The video decoding system of claim 1,wherein the processor performs a spatial down sampling of data receivedfrom the base layer decoder.
 4. A video decoding system in which videoobjects are recognized from video data, wherein instances of a videoobject at given times are coded as video object planes (VOPs) and VOPsare assigned to one or more video object layers, the video decodingsystem comprising: a base layer decoder having an input for VOP dataassociated with a first video object layer of the video object; aprocessor coupled to an output of the base layer decoder; and anenhancement layer decoder, having a first input for VOP data associatedwith a second video object layer of the video object and a second inputcoupled to the processor and responsive to bidirectionally-predictivecoded VOP (B-VOP) data including a ref_select_code included therein, theenhancement layer decoder decoding the B-VOP data with reference toforward and backward reference VOPs selected from the group of: forward:data of a VOP most recently decoded by the enhancement layer decoder;and data of a most recent VOP in display order decoded by the base layerdecoder; and backward: data of a temporally coincident VOP decoded bythe base layer decoder; data of a most recent VOP in display orderdecoded by the base layer decoder; and data of a next VOP in displayorder decoded by the base layer decoder.
 5. A tangible computer-readablestorage medium storing instructions for controlling the instructionscausing the decoder to perform steps comprising: identifying data of acoded video object based on a start code indicative thereof; identifyingdata of a plurality of coded video object layers (VOLs) based onrespective VOL start codes; identifying data of plurality of coded videoobject planes (VOPs) based on respective VOP start codes, each coded VOPbeing a member of at most one VOL; determining, from data of a firstVOL: whether scalability had been applied to the first VOL coding; andif scalability had been applied, identifying from the coded data of thefirst VOL data a second VOL that is a reference layer to the first VOL;and decoding a VOP from the first VOL with reference to a VOP of thesecond video object layer.
 6. The tangible computer-readable storagemedium of claim 5, wherein the data of the first VOL comprises, ifscalability had been applied, the following fields: a flag indicatingthe use of scalability; a parameter identifying the second VOL; aparameter indicating whether the second layer has higher resolution thanthe layer being coded; a parameter identifying a numerator of a ratio tobe used in horizontal spatial resampling in scalability; a parameteridentifying a denominator of the ratio to be used in horizontal spatialresampling in scalability; a parameter identifying a numerator of aratio to be used in vertical spatial resampling in scalability; and aparameter identifying a denominator of the used in vertical spatialresampling in scalability.
 7. The tangible computer-readable storagemedium of claim 5, wherein data of a VOP may be coded according to oneof the following techniques: intra coding, predictive coding andbidirectionally predictive coding.
 8. The tangible computer-readablestorage medium of claim 7, wherein the instructions on thecomputer-readable medium further comprise: for a predictive coded VOP(P-VOP), decoding the P-VOP data with reference to data of at most oneother reference VOP; and for a bidirectionally coded VOP (B-VOP),decoding the B-VOP data with reference to data of at most two otherreference VOPs.
 9. The tangible computer-readable storage medium ofclaim 8, further comprising, for a P-VOP that is a member of a VOL forwhich scalability had been applied, identifying the reference VOP from aref_select_code provided in the data of the P-VOP, respective values ofthe ref_select_code identifying the reference VOP as one of: a mostrecently decoded VOP in the VOL to which the P-VOP belongs; a mostrecent VOP in display order belonging to the reference layer; a next VOPin display order belonging to the reference layer; and a temporallycoincident VOP in the reference layer.
 10. The tangiblecomputer-readable storage medium of claim 9, wherein the ref_select_codeis a two bit code.
 11. The tangible computer-readable storage medium ofclaim 8, further comprising, for a B-VOP that is a member of a VOL forwhich scalability had been applied, identifying forward and backwardreference VOPs from a ref_select_code provided in the data of the B-VOP,wherein a respective value of the ref_select_code identifying theforward reference VOPs is one of: a most recently decoded VOP in the VOLto which the B-VOP belongs; and a recent VOP in display order belongingto the reference layer; and a respective value of the ref_select_codeidentifying the backward reference VOPs is one of: a temporallycoincident VOP in the reference layer; a most recent VOP in displayorder belonging to the reference layer; and a next VOP in display orderbelonging to the reference layer.
 12. The tangible computer-readablestorage medium of claim 11, wherein the ref_select_code is a two bitcode.